Light in the near-infrared (close to visible red) can penetrate surprisingly far through human tissue, a fact that is being used to "illuminate" the interior of the brain in a noninvasive technique known as near-infrared spectroscopy (NIRS). In this procedure, illustrated in FIGURE 25-48, an optical fiber carrying a beam of infrared laser light with a power of 1.5 mW and a cross-sectional diameter of 1.2 mm is placed against the skull. Some of the light enters the brain, where it scatters from hemoglobin in the blood. The scattered light is picked up by a detector and analyzed by a computer.
In figure 25-48

(a) According to the Beer-Lambert law, the intensity of light, I, decreases with penetration distance, d, as I = I0e-μd, where I0 is the initial intensity of the beam and μ = 4.7 cm-1 for a typical case. Find the intensity of the laser beam after it penetrates through 3.0 cm of tissue.
(b) Find the electric field of the initial light beam.

Transcribed Image Text:

Light from infrared laser Light detection Light emission Skull Infrared light scatters from hemoglobin in brain Cerebral cortex